1. Field
One or more embodiments relate to a luminous flux limit device, an optical scanning unit including the luminous flux limit device, and an electrophotographic image forming apparatus.
2. Description of the Related Art
An electrophotographic image forming apparatus, such as a laser printer, a digital copy machine, or a multi-functional printer (MFP), has a structure configured to form an electrostatic latent image by scanning light on a photosensitive body via an optical scanning unit, develop the electrostatic latent image into a developed image by using a developer such as toner, and transfer the developed image onto a printing medium.
Optical scanning units for use in electrophotographic image forming apparatuses may be required to have a high-definition image formation property. Furthermore, a larger tolerance of an error of an arrangement of a photosensitive body disposed near an image-forming region may be required.
In optical scanning units for use in electrophotographic image forming apparatuses, a desired optical beam spot has been achieved in an image-forming region by arranging a luminous flux limit device having an appropriate-sized optical aperture in main and sub scanning directions before a luminance flux is incident on a deflector. An optical aperture of a general luminous flux limit device may have a circular, oval, or tetragonal shape, and may not have a structure that transforms a transmission amplitude and phase of a luminous flux. When a general optical aperture is used, a wavelength of an optical system may be reduced or a numerical aperture (NA) of the optical system may be increased to increase a resolution on an image-forming surface. However, since the depth of focus of the optical system is inversely proportional to the square of the NA, the depth of focus decreases when the NA increases. Thus, an allowed tolerance of the arrangement of a photosensitive body in a focal region decreases.